1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, that is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in that each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in that each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or ant parallel to this direction.
The sequence of exposures, or shots, forms a printed pattern. When considering maskless lithography, each shot results from an image of a spatial light modulator (SLM) array being projected onto a photosensitive surface, such as a wafer substrate. This results in application of a dose, or a quantity of irradiation from a light source, within a certain exposure zone on this surface. Exposure zones are created when the substrate surface is illuminated by flashes of light from the light source. When the pattern extends beyond the boundaries of exposures of a single SLM, the exposures are stitched together along adjacent boundaries to form a completed pattern.
Stitching errors in the printed pattern occur near these boundaries between adjacent exposure zones due to both geometrical misalignments of the exposures and disturbances due to other optical phenomena. Generally, stitching errors occur in printed patterns due to spatial misalignment of the exposure zone on the wafer from its expected position. Optical effects can also create stitching errors, even in cases where the alignment can be perfect. Even a small spatial misalignment of the shots, in the case of a spatial misalignment, can result in a significant perturbation of the printed pattern near the stitching line.
The optical effects can be due to the fact that distribution of the dose within each exposure zone is a result of an exposure by partially coherent light. Since two adjacent exposure zones are exposed at different times, the exposures are effectively incoherent, thus creating the unwanted optical effects. In the example of exposing a substrate to form a flat panel display (FPD), the pixel grid imaging technique used is fully incoherent. The optical errors are caused by different transmission ratios of the optical path for the different regions. A certain area can be brighter than another.
Known techniques attempt to compensate for stitching errors in printed patterns that occur near the stitching line between adjacent exposure zones. A first such technique does not utilize overlap of exposure areas, and involves providing assist features during second, or subsequent, passes of the exposure process. The assist features are added to the pattern data used in irradiating the photosensitive surface, and fill up areas that might otherwise not have received sufficient doses of radiation.
Alternative techniques include compensation spanning overlapping areas of adjacent exposure zones. For example, the SLM can be adjusted, such that a feature to be imprinted on the overlapping region of the photosensitive surface is only printed during one of the multiple exposures for that region. That is, pixels of the SLM that might otherwise be switched “on” during a given exposure to expose a feature are switched “off” by a control system that has determined that an alternative exposure also covering that region will expose that feature instead.
However, such known techniques for compensating for stitching errors inevitably create an area of stitching that will appear different to the no stitched areas of the display. These techniques also require that the substrate be positioned with a very high accuracy in order to align adjacent exposure zones. The dose provided by the illumination source must also be very tightly controlled.
Therefore, what is needed is a system and method to substantially reduce, and possibly eliminate, visibility of a stitched areas and/or to reduce constraints on positioning of a substrate and a dosage control of an illumination source.